two_pole_resonator.hpp

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#pragma once
 
#include <pulp/signal/denormal.hpp>
 
#include <algorithm>
#include <cmath>
 
namespace pulp::signal {

template <typename SampleType = float>
class TwoPoleResonatorT {
public:
    TwoPoleResonatorT() { update(); }
 
    void set_sample_rate(double sr) {
        sample_rate_ = sr > 0.0 ? sr : sample_rate_;
        update();
    }
 
    void set_frequency(double hz) {
        // Above Nyquist the recurrence is still stable but the resonance folds,
        // so clamp rather than let a pitch envelope sweep into nonsense.
        frequency_ = std::clamp(hz, 0.0, 0.499 * sample_rate_);
        update();
    }

    void set_t60_ms(double ms) {
        t60_ms_ = std::max(ms, 0.01);
        update();
    }
 
    double frequency() const { return frequency_; }
    double t60_ms() const { return t60_ms_; }
 
    void reset() {
        y1_ = 0;
        y2_ = 0;
    }

    bool is_ringing() const {
        return std::fabs(static_cast<double>(y1_)) > 1e-7 ||
               std::fabs(static_cast<double>(y2_)) > 1e-7;
    }
 
    SampleType process(SampleType excitation) {
        const SampleType y = static_cast<SampleType>(a1_) * y1_ -
                             static_cast<SampleType>(a2_) * y2_ +
                             static_cast<SampleType>(gain_) * excitation;
        y2_ = y1_;
        y1_ = snap_recursion_state(y);
        return y;
    }
 
private:
    void update() {
        // T60 in samples -> pole radius: the radius raised to that many samples
        // must equal -60 dB.
        const double samples = std::max(0.001 * t60_ms_ * sample_rate_, 1.0);
        radius_ = std::pow(10.0, -3.0 / samples);
        const double theta = 2.0 * 3.14159265358979323846 * frequency_ / sample_rate_;
        a1_ = 2.0 * radius_ * std::cos(theta);
        a2_ = radius_ * radius_;
        // Floor the gain so a body tuned to DC still passes its excitation
        // rather than muting it.
        gain_ = std::max(std::sin(theta), 1e-4);
    }
 
    double sample_rate_ = 44100.0;
    double frequency_ = 100.0;
    double t60_ms_ = 300.0;
 
    double radius_ = 0.0;
    double a1_ = 0.0;
    double a2_ = 0.0;
    double gain_ = 1.0;
 
    SampleType y1_{};
    SampleType y2_{};
};
 
using TwoPoleResonator = TwoPoleResonatorT<float>;
using TwoPoleResonator64 = TwoPoleResonatorT<double>;
 
}  // namespace pulp::signal